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1.1 Biochemistry

The World Health Organisation (WHO) encourages breastfeeding up to and beyond 2 years - after 6 months of exclusive breastfeeding, other nutritionally adequate and safe foods may be introduced.1 Exclusive breastfeeding provides all the nutrients and water that an infant needs to grow and develop in the first six months.

The composition of breastmilk is relatively constant with minimal fluctuations caused by maternal diet. Unlike the nutrition received by the fetus through the placenta, the nutrition received by breastfed infants is not dependent on the status of maternal metabolism. The mechanisms that cause breastmilk to be synthesized are insulated from variations in maternal nutritional intake, ensuring that sufficient milk of adequate composition is available to the infant even during inadequate food intake by the mother.2

The infant will more than double his birth weight during the first 6 months, a time when he is fed on a diet exclusively of his mother's breastmilk. Maternal metabolism is adjusted to redirect nutrients to the breast to meet this additional need. However, for her own well-being a well-nourished woman should aim to consume an additional 500 kcal per day in the form of nutritious snacks.3 Additional fluid requirements are met by recommending drinking sufficient water to avoid thirst. Consuming volumes of water in excess to needs will NOT increase breastmilk production.4,5

a. Protein

Human milk has a low protein content, approximately 9g/L, decreasing as lactation progresses.6 This is less than measured protein in bovine milk, however it is of higher biological value and perfect for a human infant.7

Whey proteins form the predominant proteins in breastmilk - approximately 60%. Casein forms the remaining 40%. These values are approximate... some texts showing the ratio may be between 50:50 (in very late lactation) to 80:20 at different stages of lactation, with whey predominating.

The high concentration of whey proteins are digested quickly and easily in the infant's stomach. Breastmilk casein has a mainly nutritive function, providing minerals and essential amino acids to the infant and forms a soft, flocculent curd during digestion. Bovine casein (the predominant protein in cows' milk) forms a tough, less digestible curd.

You will be aware that proteins have significant nutritional properties. Proteins in breastmilk also have other functions such as:

  • essential amino acids for growth
  • protective factors (eg. immunoglobulins, lactoferrin, etc.)
  • carriers for hormones (eg. thyroxine, cortisone-binding proteins)
  • carriers for vitamins (eg folate, Vit D, Vit B12 binding proteins)
  • enzymatic activity (eg amylase, lipase)
  • growth factors (eg insulin-like growth factor, epidermal growth factor)

Did you know...?

The high concentration of whey proteins and the soft, flocculent curds formed by the casein is the reason why breastfed babies do not get constipated. If a mother is concerned her baby might be constipated it is important to investigate the cause, because this is NOT normal for the (exclusively) breastfed baby.

  • The proteins of cow's milk, goat's milk and soy beans (all used to make artificial infant formula) are different in structure, quantity and quality to the proteins in breastmilk.
    • allergic reactions commonly occur
    • any protective factors that remain after manufacture will not function in the same way they do from human milk
    • the baby's immune system will not be supported as it gradually matures

Below is the first of your Workbook Activities. Have you printed your Workbook yet? If not, return to the course information page, click on the link and print the Workbook. Fill out the activities as you progress through this course.

Workbook Activity 1.1

Complete Activity 1.1 in your workbook.

b. Carbohydrates

Lactose (milk sugar) is the principle carbohydrate in human milk. Lactose is the most stable component of mature breastmilk. Average concentration is 68g/L (some texts report 70g/L). Lactose is synthesized in the mother's breast and broken down by the enzyme lactase in the baby's small intestine. Lactase breaks lactose into glucose and galactose, ready for absorption into the blood stream.

The role of lactose:

  • The rapid increase of lactose levels in colostrum at lactogenesis II causes osmotic drawing of water into the breast secretion resulting in copious breastmilk production.
  • Provides energy to the body - and particularly to the rapidly growing infant brain.
  • Enhances absorption of calcium and iron.
  • Galactose is ultimately essential for development of central nervous system.

There have been around 130 different oligosaccharides (short chains of sugar molecules) identified in human milk.8 These important sugars comprise up to 1.2% of mature human milk, compared to only 0.1% of bovine milk. Their role is in protection of the infant from infections.

Food for thought...

At a concentration of 70g/L, human milk has the highest concentration of lactose of all the mammalian milks. Have you ever wondered why? Could it be because the human brain has the MOST growth of all mammal species to accomplish over the next two years and lactose contains elements essential to brain growth?

Consider what effects a lactose-free artificial infant formula could have on the infant fed on it. Discuss this with your colleagues.

Workbook Activity 1.2

Complete Activity 1.2 in your workbook.

c. Milk Lipids (fats)


Properties of milk lipids:
  • provide around half of the energy (kilojoules) in breastmilk
  • 97 - 98% of milk lipids are triglycerides
  • essential for the synthesis and development of retinal and neural tissues
  • are a rich source of the essential fatty acids linoleic acid and alpha-linolenic acid and their long-chain derivatives arachidonic acid (AA) and docosahexaenoic acid (DHA). DHA is essential for the developing visual system.

    Milk lipids are the most variable constituent of human milk.

    The change in fat concentration in breastmilk is directly related to the amount of milk held in the breast at that time. Milk removed when the breast is fullest has a low concentration which increases in a linear fashion as more and more milk is removed. Variations in fat content also occur with time of day, stage of lactation, parity, age, and between women.9


    © Dr Jacqueline Kent, Biochemistry and Molecular Biology, The University of Western Australia

    Illustrated above is a series of samples from an expression collected in 1mL fractions. The samples are, in order from left to right, a fore-milk sample (hand-expressed), a stimulation sample (the first milk removed by the breast pump), 7 samples collected during the expression, and a final sample hand-expressed after the expression. The initial sample is 5.6% cream (fat) and the final is 18.3% cream (fat).

    The fat is seen as small clumps of white towards the top of each tube - the amount of fat increases as the breast is progressively drained.

    For this mother this represents a change in degree of fullness from 0.55 (about half full) to 0.0 (well-drained).10

    Workbook Activity 1.3

    Complete Activity 1.3 in your workbook.

    Foremilk vs Hindmilk

    Many misunderstandings surround the use of these terms. The milk available at the beginning of a breastfeed is sometimes known as foremilk, and hindmilk is often used to describe the milk consumed by the infant at the end of the breastfeed.

    The only difference between foremilk and hindmilk is in fat content, with foremilk having a lower fat content than hindmilk at a given breastfeed. You have just learned that the fat content increases in a linear fashion as milk is removed from the breast during that breastfeed, therefore, the change from foremilk to hindmilk is not defined. Use of the terms 'foremilk' and 'hindmilk' may be misrepresentative.

    Breastmilk storage capacity

    Breastmilk is stored in the alveoli of the breast, and storage capacity of the breast is unique for every woman and every breast. Some mothers may have a large storage capacity, while others may have a very small capacity, and most women have differing capacity in each breast.11,12,13 You cannot accurately judge a woman's breast capacity visually - don't assume that a large breasted woman has a large capacity.

    For a mother who has a large storage capacity the milk received by her baby while the breast is at its fullest will be low in fat. After breastfeeding several times the volume in the breast will be reduced and the fat content of subsequent feeds will be much higher. However, for the mother with a small storage capacity, her baby may remove all or most of the breastmilk at most feeds. The fat content of breastmilk at each feed will be similar.

    Clinical Tip

    Mothers should follow their baby's feeding cues. The mother with the smaller breastmilk storage capacity will find her baby will cue to feed frequently. The mother with the larger storage capacity may find her baby takes larger feeds and requests fewer feedings. Both babies may consume similar amounts of breastmilk over a 24-hour period and both will grow equally as well. Scheduling feeding may work for some babies but cause other babies to be very unhappy and cause failure to thrive.

    Cholesterol

    The level of cholesterol in breastmilk remains constant (10-20mg/dL) despite dietary manipulation of the mother's cholesterol intake. There is negligible cholesterol in artificial infant formula.

    Cholesterol is required to build and maintain cell membranes. Amongst other important tissues it is involved in laying down the myelin sheath which covers the axons of nerve cells in the rapidly growing brain and spinal cord. Multiple sclerosis, a problem of myelinisation, is much more prevalent in countries where artificial infant feeding is common.14

    The high level of cholesterol in breastmilk appears to have a 'programming' effect on infants, protecting them from detrimental effects in later life, with adults who were artificially-fed having significantly higher total cholesterol levels and incidence of coronary heart disease.15,16,17

    d. Vitamins

    Vitamin A
    • necessary for vision and maintenance of epithelial structures
    • adequate stores laid down in the fetal liver during the last trimester
    • human milk is an excellent source of vitamin A
    Vitamin D
    • synthesized in the skin from cholesterol on exposure to UVB radiation (sunshine)
    • stimulates intestinal absorption and renal reabsorption of calcium and phosphorus
    • involved in bone resorption and bone formation
    • fetal stores of Vitamin D in infants born to mothers with normal status may be depleted by 2 months of age in the absence of any exposure to sunlight
    • breastmilk is a negligible source (20 IU/L) for a recommended need of 300 - 400 IU per day
    • very dark skinned breastfed infants or those infants not exposed to adequate sunlight may require oral supplementation
    Vitamin E
    • mature human milk meets the daily recommended intake
    Vitamin K
    • prothrombin, coagulation Factors VII and IX and some plasma proteins are vitamin K-dependent proteins. These are blood clotting factors.
    • Vitamin K (phylloquinone) levels in human milk vary considerably depending on maternal diet. Maternal supplementation of 5mg/day increases breastmilk concentration to levels which provide the infant's daily requirement.
    • Vitamin K synthesis by bacteria in the large intestine in the first week of life provides insufficient levels for the fully breastfed infant because the predominant gut bacteria (bifidobacteria) does not synthesize Vitamin K.
    • Once only intramuscular injection of Vitamin K is recommended for all infants at birth. No other supplementation is required.
    Vitamin B
    • Most B vitamins are in appropriate concentrations in breastmilk irrespective of maternal intake. Precaution should be taken with Vitamin B6 as mega-doses (ie 600mg/day) have been shown in some studies to reduce maternal prolactin levels. Usual supplement is 25mg per day. Mothers with a long-term history of oral contraceptive use may be deficient in Vitamin B6.
    • A strict maternal vegan diet without B12 supplementation has resulted in serious infant morbidity.18,19 Likewise mothers with gastric bypass surgery are also at risk of Vitamin B12 deficiency.20
    Vitamin C
    • Vitamin C levels in breastmilk remain within a normal range, regardless of maternal supplementation.

    e. Minerals

    Concentration of minerals in human milk appears to be quite low, however they have a very high bioavailability and their interrelationship with other nutrients may affect their absorption, metabolism and excretion.

    Calcium

    Lactating women are often advised to take a calcium supplement or to increase, above normal, their intake of calcium-rich foods. However, in a large study of a group of women in Cambridge UK21 there was no correlation found between calcium intake (ranging from 600 to 2300mg/day) and the amount of calcium in their breastmilk.

    The infant's daily requirement for calcium is adequately met by breastmilk. Bone growth in the infant is unaffected by maternal supplements.

    Maternal bone mineral density is not affected adversely by breastfeeding, or enhanced by calcium intake above normal levels. Within 3 months of weaning bone mineral density in breastfeeding women has returned to normal, or is even enhanced21,22. Breastfeeding decreases incidence of osteoporosis in post-menopausal women23 and parity with prolonged total duration of breastfeeding has no detrimental effect on bone mineral density.24

    Iron

    • levels relatively low but highly bioavailable to infant - five-fold more efficient absorption from human milk than from bovine milk25
    • heat treatment of breastmilk does not alter the iron-absorption rate
    • the presence of high lactose and Vitamin C levels in breastmilk also aid its absorption
    • iron supplements for term infants during the first 6 months of exclusive breastfeeding is unnecessary. Complementary foods after 6 months of age should include iron-rich foods. Some infants who continue to be exclusively breastfed for much longer than 6 months may maintain an adequate iron status - biochemical analysis on an individual basis may be indicated.26

    Iodine

    • required for synthesis of thyroid hormones that are required for brain development during fetal and early postnatal life
    • iodine deficiency is a leading cause of brain damage
    • breastmilk levels vary widely according to geographic region and maternal intake
    • supplementation of women during pregnancy and lactation in iodine-deficient areas will reverse this leading cause of mental impairment25

    f. Water

    Breastmilk contains a high percentage of water. When babies have unrestricted access to the breast they DO NOT need additional water, even in hot climates. Giving water or other fluids such as teas, will decrease the infant's desire to breastfeed decreasing nutrient intake and breastmilk synthesis, and increasing the infant's risk of infections.

    The taste of breastmilk

    The flavour (flavor) of breastmilk is affected by the foods in the maternal diet. This daily variation in flavour can help the infant to become used to the tastes of the family foods and ease the transition to these foods after six months of age. Artificial infant formula tastes the same for every feed. The taste of formula is not related to any foods the baby will eat when older.

    Dietary advice

    What should I remember?

    1. The quality of breastmilk is NOT dependent on maternal diet.
    2. Proteins: several functions, including immunological protection.
    3. Carbohydrates: provide energy and support brain development.
    4. Lipids: most variable; provide energy; support retinal and brain growth.
    5. Vitamins and minerals have high bioavailability. Supplementation is not necessary. Exceptions: vitamin B12 in strict vegans; iodine in iodine-deficient areas; vitamin D when exposure to sunlight restricted.
    6. Additional water is not required.
    7. Breastmilk meets all of the infant's nutritional needs for the first 6 months.

    Self-test Quiz

    Notes

    1. # WHO (2002) Global Strategy on Infant and Younf Child Feeding
    2. # Neville MC (2001) Anatomy and Physiology of Lactation
    3. # Picciano MF (2003) Pregnancy and Lactation: Physiological Adjustments, Nutritional Requirements and the Role of Dietary Supplements
    4. # Dusdieker LB et al. (1990) Prolonged maternal fluid supplementation in breast-feeding.
    5. # Dusdieker LB et al. (1985) Effect of supplemental fluids on human milk production.
    6. # Riordan J (2005) Breastfeeding and Human Lactation
    7. # Hale TW et al. (2007) Textbook of Human Lactation
    8. # McVeagh P et al. (1997) Human milk oligosaccharides: only the breast
    9. # Daly SE et al. (1993) Degree of breast emptying explains changes in the fat content, but not fatty acid composition, of human milk
    10. # Kent J (2005) Personal communication
    11. # Engstrom JL et al. (2007) Comparison of milk output from the right and left breasts during simultaneous pumping in mothers of very low birthweight infants.
    12. # Ramsay DT et al. (2005) Anatomy of the lactation human breast redefined with ultrasound imaging
    13. # Cox DB et al. (1997) Studies on Human Lactation: The Development of the Computerized Breast Measurement System
    14. # Pisacane A et al. (1994) Breastfeeding and multiple sclerosis
    15. # Das UN (2003) A perinatal strategy to prevent coronary heart disease
    16. # Martin RM et al. (2005) Breastfeeding and atherosclerosis: intima-media thickness and plaques at 65-year follow-up of the Boyd Orr cohort
    17. # Owen CG et al. (2008) Does initial breastfeeding lead to lower blood cholesterol in adult life? A quantitative review of the evidence.
    18. # Codazzi D et al. (2005) Coma and respiratory failure in a child with severe vitamin B12 deficiency
    19. # Allen LH (2008) Causes of vitamin B12 and folate deficiency.
    20. # Grange DK et al. (1994) Nutritional vitamin B12 deficiency in a breastfed infant following maternal gastric bypass
    21. # Prentice A (2000) Calcium in pregnancy and lactation
    22. # Kovacs CS (2005) Calcium and bone metabolism during pregnancy and lactation
    23. # Schnatz PF et al. (2010) Effects of age at first pregnancy and breast-feeding on the development of postmenopausal osteoporosis.
    24. # Lenora J et al. (2009) Effects of multiparity and prolonged breast-feeding on maternal bone mineral density: a community-based cross-sectional study.
    25. # Picciano MF (2001) Nutrient Composition of Human Milk
    26. # Griffin IJ (2001) Iron and Breastfeeding